There exists a confocal scanning microscope as a technology for improving the resolution of the optical microscope. There are two types of the optical system: a reflective type and a transmission type. For better understanding of the optical system, the microscope of the transmission type will be explained. FIG. 8 is a schematic diagram of a transmission type optical system. Light from a light source 101 is irradiated to a pin hole 205 to get a point light source. In reality, the point light source is not a perfect point light source, since a pin hole of a finite size is used. A light from the pin hole is focused onto an observation object 202 through an objective lens 201. The observation object 202 can be scanned in a three dimensional direction by a scanning mechanism 102 such as a voice coil. The light, which has passed through the observation object, passes through a detection lens 203 and is focused onto a pin hole 204. The light, which has passed through the pin hole 204, is detected by a light detector 103, and its signal is displayed on a display device 104 configured to display an image associated with a scanned position of the observation object. It is known that lateral resolution of the confocal scanning microscope depends on the size of the pin hole 204. Smaller the pin hole, higher the resolution. Reversely, larger the pin hole, lower the resolution and closer to a resolution of a conventional optical microscope which is not of the scanning type. The conventional optical microscope herein referred is a non-scanning type microscope which irradiates a wide range of an observation object and forms an image from a light which has passed through an objective lens from the observation object. FIG. 9 shows point spread functions of the conventional optical microscope and a confocal scanning microscope having a pin hole of ultra minimum size. Abscissa v is expressed in a normalized optical unit defined as v=2 π·x·NA/λ, in which NA represents the number of apertures of the objective lens 201 and the detection lens 203, and both of the lens are assumed to have a same NA. Further, x represents a coordinate vertical to the optical axis or a lateral coordinate, and λ represents the wavelength of light. A point spread function 131 of the confocal scanning microscope is narrower than a point spread function 130 of the conventional optical microscope, from which it is understood that the resolution has been improved.
A non-patent document (C. J. R. Sheppard and A. Choudhury, “Image Formation in the Scanning Microscope”, Opt. Acta, Vol. 24, 1051-1073 (1977)) discloses a method of shielding a central portion of the beam for further improving the resolution. In an optical system of the confocal scanning microscope shown in FIG. 10, a circular light shielding plate 229 having a center thereof on the optical axis is inserted in an optical path of the light which has passed through an observation object 202. The circular light shielding plate 229 disposed in such a manner eliminates low frequency components in a spatial frequency region of the light emitted from the observation object 202, and thereby improves the resolution.